you gotta realize reddit didn't just "appear" one day with those obscure niche topics built out. There is a network effect large communities have. We need hundreds of thousands more members before that is possible.
I think you probably weren't there for early reddit, but most of the active posters here on Lemmy were. It was tiny. Like Lemmy.
You can't force those niche communities to exist here. It doesn't work. But what you can do is post and create valuable content. and eventually we may get there.